Generally, a solenoid consists of an iron housing enclosing a coil and an armature which is movable within the coil between an extended position where the armature extends from one end of the coil and a retracted position where the armature is within the coil. When an electrical current flows through the coil, a magnetic filed is established which is used to draw the armature inside of the coil until it makes contact with an end of the housing. The force exerted on the armature due to the magnetic field, and in turn, the force which the solenoid is capable of exerting is related to the position of the armature within the coil. As the armature moves into the coil, it replaces what would otherwise be an air gap, and thereby tends to concentrate the lines of magnetic flux forming the magnetic field. This is due to a change in the permeance (i.e. the ease with which the magnetic flux passes) within the magnetic circuit existing within the solenoid. This change in the permeance causes an exponential increase in the force exerted on the armature as it is drawn into the coil. Since the armature weighs only a few ounces, it is rapidly accelerated by this exponentially increasing force and can reach a very high velocity. The high velocity which is reached causes the armature to impart a considerable mechanical vibration within the solenoid as the armature reaches the end of its stroke and contacts the end of the housing. This vibration results in the production of a noise, which in sound reproducing mechanisms such as tape recorders, is very undesirable.
Devices have been used to limit the maximum velocity reached by the armature, and accordingly the amount of vibration produced. One such device is a spring placed in opposition to the movement of the armature, which is compressed by movement of the armature when the solenoid is activated. This spring exerts a counter force on the armature which is generally proportional to the amount of compression. Once compressed, the spring force does not disappear, however and current must continue to flow through the coil of the solenoid, or the armature will be returned to its extended position by the spring force. This continued current flow produces heat which must be dissipated or the solenoid might over heat. For this reason, solenoids in which a spring is used for limiting the armature velocity typically also include a mechanical latching device which latches the spring in its compressed position in order to eliminate the counter force exerted by the spring after the armature reaches its retracted position. This additional latching device allows the current flow in the solenoid to be reduced without the armature returning to its extended position.
Other solenoids utilize an air or fluid pot as an alternative to a spring, which air or fluid pot is a variable volume cylinder (e.g. a piston within a cylinder) containing air or fluid which is allowed to escape at a controlled rate through an orifice within the cylinder, as the movement of the armature forces the piston to move into the cylinder. The counter force exerted by such an air or fluid pot is determined by the area of the orifice through which the air or fluid is escaping, and the pressure against which the air or fluid must flow. An air or fluid pot is more costly than its mechanical counterparts because of the required close tolerances involved with hydraulic devices, and the need to prevent leakage of air or fluid from the hydraulic system.